A large portion of the population experience difficulty with their vision due to a number of possible conditions. The majority of vision problems in this portion result from a condition known as myopia, or nearsightedness. Myopia is a common condition where an eye cannot focus on far objects because the cornea of the eye is curved too steeply to provide adequate focusing at the retina of the eye. Alternatively, the eye may be afflicted with a condition know as hyperopia, or farsightedness. With hyperopia, the eye cannot focus on near objects because the cornea of the eye is curved too flatly to provide adequate focusing at the retina of the eye. Another common condition is astigmatism, where unequal curvature of one or more refractive surfaces of the cornea prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision.
Conventional contact lenses with a longer central radius of curvature than the central radius of the cornea are known to change the shape of the cornea by compressing the surface at its apex. This "reshaped cornea" has a lengthened radius of curvature in its central zone, which serves to improve myopia, and to generally improve visual acuity. The procedure for obtaining this sort of correction to vision, where correction persists for some time after removal of the corrective lenses, is commonly known as orthokeratology.
Orthokeratology has been performed in some form or another since the early 1970's. Unfortunately, the time needed to achieve a desired visual correction using conventional orthokeratology can range from one to two years. Moreover, conventional orthokeratology is typically limited to correcting only 1.50 diopters of myopia, and the length of time in which this correction would "hold" before degrading is extremely variable, which regrettably necessitates wearing a retainer lens for at least part of the day.
Recent advances in orthokeratology have generated improvements in the amount of correction available. Present orthokeratology lenses can now typically correct up to 3.0 diopters of myopia. Furthermore, the time required to achieve correction has dropped from several years to several months on low to moderate myopes. However, the holding power associated with these improved orthokeratology treatments remains a problem. Even the best presently available orthokeratology treatment plan requires a patient to wear a retainer lens for some portion of their daytime or nighttime vision requirements, and in problem cases, possibly wear a retainer lens all the time to maintain corrected vision.
Thus, what is needed is a contact lens and corresponding orthokeratology treatment method that corrects substantial amounts of myopia, hyperopia, and astigmatism, over a short treatment period, and with substantial holding power that allows correction to last throughout a desired period of daytime and nighttime corrected vision.